This "relatively" simple idea has been bouncing around by brain for a few days now and, I thought, may prove useful for the wider Sandboxie community.

When browsing the Internet with my sandboxed browser, I have found that sometimes my browser gets high-jacked by a rogue program that opens a new browser tab, takes control of the entire browser, cannot be exited, deleted or moved away from unless this program's download is triggered.

With sandboxie, I click the "Terminate All Programs" menu option and put an end to this nonsense but I do have to start my browsing all over again. I was wondering if it is possible that Sandboxie can be updated with an additional feature, i.e. "Terminate Tab x" where X is a number counting from left to right, while still keeping "Terminate All Programs" as a final option.

In other words, give Sandboxie an additional ability to terminate a specific browser tab without closing down the entire browser or browsing session.

I do not know how difficult this will be, judging by the number of available browsers out there, or whether my idea has any merit but I hope someone can spend 5 or so minutes looking at it.

You can close the program that affects your browser by selecting it via Sbie Control and choosing the "Terminate program" option.
Another approach would be to restrict your Sandbox so that only allowed programs can Start:https://www.sandboxie.com/RestrictionsSettings#startrun